There is a baseline skip before the first paragraph and between the first and second paragraphs. I can't find anything in the documentation that hints at this. In fact, on page 248 (Manual for Version 3.0.1a) there is this for overlay: "The effect of this option is that everything within the current scope is not taken into consideration when the bounding box of the current picture is computed." That led me to believe that the option would be spacing-neutral.